1. Field of the Invention
This invention generally relates to a sensor device such as a gyro sensor or an acceleration sensor used for detecting angular velocity on a car navigation system or the like, and more particularly, to a stem structure of the sensor device. The present invention also relates to a manufacturing method of the sensor device.
2. Description of the Related Art
Conventionally, acceleration sensors and angular velocity sensors (gyro sensors) having various types of configuration have been proposed. Japanese Patent Application Publication No. 6-58369 (hereinafter referred to as Document 1) describes an acceleration sensor shown in FIG. 1. The acceleration sensor 1 includes a base plate 11, a sensor chip 3, a cap 8, and lead terminals 5. The sensor chip 3 is mounted on a circuit board provided on the base plate 11. The cap 8 seals the sensor chip 3. The lead terminals 5 penetrate the base plate 11. Additionally, the acceleration sensor includes electronic components 4 and lead wires 7 on the circuit board. The lead wires 7 connect the circuit board and the lead terminals 5.
The base plate 11 forms a stem of the acceleration sensor and includes a dent portion 13 and a step portion 12. The above-mentioned base plate 11 is formed with embossing press (folding press). That is to say, the dent portion 13 is formed by pressing up a plane plate, and then the step portion 12 is formed by pressing up from the opposite side of the plane plate. The cap 8 is attached to the base plate 11 by hermetically sealing a flange 9 of the cap 8 with a circumference of the base plate 11 by resistance welding, with fitting together with the step portion 12 of the base plate 11.
Document 1 discloses that the cap 8 is attached by the resistance welding; however, the detail is not described. It is known that there are various kinds of resistance welding, and for instance, projection welding is preferable in order to achieve an excellent hermetic sealing. However, the base plate 8, to which the flange 9 of the cap 8 is welded, is plane, so the projection welding cannot be employed.
The base plate 11 is formed by embossing press. This makes it impossible to obtain a sufficient height of the step portion 12 to serve as a protective barrier against welding spatters.